1. Field of the Invention
The present invention is directed to Time Division Multiple Access (TDMA) telecommunication systems and more particularly to a method of conveying information in such systems where the information is arranged in a new format.
2. Description of the Related Art
Wireless telecommunication systems use various schemes to allow multiple users to use (i.e., share simultaneously) a particular bandwidth. The bandwidth is part of or all of the band of frequencies allocated to the telecommunication system for conveying (i.e., transmitting and receiving) information. The information includes user information (e.g., voice) and signaling information. The signaling information are data generated by various equipment of the telecommunication system that are used by the telecommunication system to manage, process and convey the user information. One multiple access scheme that is widely used in many wireless telecommunication systems is called Time Division Multiple Access.
A TDMA wireless telecommunication system allows multiple users to use the same bandwidth by creating time frames (hereinafter xe2x80x9cframesxe2x80x9d) comprising time slots (hereinafter xe2x80x9cslotsxe2x80x9d) within which each user is allowed to convey their information. Signaling information is also conveyed during each slot. Each slot or a group of slots are defined and are assigned to a particular user. Thus, each user is allowed to transmit and/or receive information in that user""s assigned slot. In this manner, many users are able to use the same bandwidth thus increasing the user and/or information capacity of the system.
FIG. 1 depicts a typical scenario in a TDMA system where user 1 is conversing with user 2. For the sake of clarity, only two users are shown. In an actual system, there is usually more than one base station and many more users throughout the system some of whom obtain access to base station 106 via a wireline telephony network such as the Public Switched Telephone Network (PSTN). The users have mobile equipment 100 and 112 (e.g., cellular phone) which they use to communicate with each other. Base station 106 represents part of the TDMA system""s equipment which relays the user information in accordance with the signaling information and protocol used by the system. The protocol is a particular set of rules by which the system equipment and the user equipment initiate communication, convey information and terminate communication. Typically, protocols are established communication standards that are well defined and which are followed by many wireless telecommunication systems. Wireless communication channels 102 and 110 are commonly referred to as the uplink, which is the channel used by the users to transmit information to base station 106. Communication channels 104 and 108 are commonly referred to as the downlink, which is the channel used by the users to receive information from base station 106. The information (user and signaling) is conveyed over the uplink and downlink in the form of Radio Frequency (RF) signals.
FIG. 2 depicts a particular frame format and a slot format used by a protocol referred to as the IS-136 standard. The IS-136 standard is a protocol used by many TDMA wireless telecommunication systems in North America and South America. The frame format is the particular arrangement of the slots within frame 200 and the slot format the arrangement of user information and signaling information within each slot. In particular, frame 200 has a time duration of 40 milliseconds and is divided equally into six slots. IS-136 allocates 30 KHz of bandwidth per carrier. The information rate (i.e., symbol rate) is 24.3 K symbols/sec. Each slot has a time duration of 6.67 milliseconds and comprises 324 bits. Although the frame format (i.e., six slots per frame) is the same for the uplink and the downlink, the slot formats are different. Slot 202 depicts the uplink slot format and slot 204 depicts the downlink slot format. Slots 202 and 204 each are divided into fields which are portions of the slot reserved for either user information or signaling information. For example, slot 204 has fields 206 and 208 labeled DATA each of which reserves 130 bits for user information. Field 210 of slot 204 has 28 bits reserved for signaling information referred to as SYNC. The significance of the different fields of signaling information of slots 202 and 204 is defined in the IS-136 standard, TIA/EIA IS-136.1, 136.2 which is incorporated herein by reference.
Still referring to FIG. 2, for downlink slot 204, the first 28 bits of information transmitted and received (i.e., conveyed) are the SYNC bits. The next 12 bits transmitted and received are signaling bits called Slow Associated Control Channel (SACCH) bits followed by 130 bits of DATA. The remainder of the information is transmitted in the order shown. The uplink information is transmitted and received in the order shown by slot 202 of FIG. 2.
In IS-136, a maximum of either three (3) users or six (6) users are allowed to convey information within the frame and slot format discussed above. The number of users per frame depends on the mode at which the TDMA system is operating. In IS-136 compliant systems there are two modes of operation. The first mode of operation is referred to as the Full Rate mode in which each user is assigned two slots per frame. The second mode of operation is called the Half Rate mode in which each user is assigned one slot per frame. Thus, there can be up to six users per frame in the Half Rate mode and up to three users per frame in the Full Rate mode or a combination of both modes can be used in one system.
FIG. 3 depicts a user allocation scheme used in IS-136 Full Rate mode (Frame 300) and Half Rate mode (Frame 302) for the uplink and the downlink. In the Full Rate mode, user 1 is assigned to slots 1 and 4, user 2 is assigned to slots 2 and 5 and user 3 is assigned to slots 3 and 6. In the uplink and downlink slot formats, the SYNC fields are used to allow the user information (i.e., DATA) to align and synchronize a user""s information to the proper slot. Thus, the SYNC fields assist the system to delineate the slots. Each SYNC field has a bit pattern unique to the slot within which it is located.
Prior to being transmitted over a wireless communication channel, the user and signaling information are processed to attenuate adverse effects on the information caused by channel noise and other channel distortions. In particular, the RF signals, which carry the user and signaling information, are exposed to various well known distorting effects such as phase jitter, frequency offset, amplitude and phase fading and multipath distortions.
TDMA signals, especially uplink signals, are by nature bursty signals; that is the signals for any particular user are transmitted and received as short intermittent bursts each of which carries a great deal of information. When a transmitted or a received burst of information is adversely affected by channel distortions, a great amount of the information is lost. Therefore, to reduce the likelihood of losing large amounts of information, a certain amount of processing of the information is done prior to transmission. The intent of the processing is twofold; first it is to introduce redundancy to the information with the use of well known channel coding techniques. Channel coding protects information from errors by selectively introducing redundancies in the conveyed information. The second intent of the processing is to add time diversity to the information by spreading out in time the burst of information using the well known technique of interleaving. Interleaving is used to obtain time diversity in a communication system. Since channel coding techniques are designed to protect against channel errors that occur randomly or in a bursty manner, interleavers scramble the time order of the information to achieve time diversity and improve the performance of the coding scheme.
Referring to FIG. 1A, there is shown some of the processing performed on a voice signal by equipment such as base station 106 and mobile equipment 100 and 112 for a TDMA system that complies with the IS-136 standard. A speech signal is digitized (not shown) and is fed to vocoder 140. Vocoder 140 is an Algebraic Code Excited Linear Predictive (ACELP) system that represents speech signals as a mathematical model comprising various parameters. The speech is essentially converted to a group of parameters. Vocoder 140 complies with IS-641 which is a standard for ACELP vocoders. The output of vocoder 140 is fed to channel coder 142. Channel coder 142 allows the system to determine whether information transmitted over a communication channel contains errors; the channel coders also provide techniques to correct the detected errors by introducing repetitions to the vocoder output signal. Channel coder 142 thus adds error detection codes and error correction codes to the voice parameters produced by vocoder 140. The output of channel coder 142 is fed to data encryption device 144 which provides user privacy by encrypting the coded mathematical parameters. The output of encryption device 144 is fed to 2-slot interleaver 146 which spreads out a burst of information over two nonconsecutive slots. The output of interleaver 146 is fed to modulator 148. Modulator 148 converts every two bits of the interleaved coded and encrypted mathematical parameters to one symbol. The symbols are transmitted over the communication channels (uplinks 102, 110 and downlinks 104 and 108) in the form of RF signals by radio equipment (not shown).
Referring to FIG. 4, there is shown the 2-interslot interleaving process done for an IS-136 compliant TDMA system in the Full Rate mode. Two storage devices are used to store information for transmission. Storage device 402 is divided into two buffers (buffer 402A and buffer 402B) each of which has a capacity of 10 milliseconds worth of information. At the beginning of slot 1 (i.e., at time t1) buffer 402A is filled with 10 milliseconds worth of information and buffer 402B is also filled with 10 milliseconds worth of information. Storage device 404 is empty. During the occurrence of slot 1, the 10 millisecond of information in buffer 402A is transmitted. Between time t1 and t2 bufffer 404A is filled with 10 milliseconds worth of information and buffer 404 B is also filled with 10 milliseconds worth of information. A time t2, the contents of buffer 404A and 402B are transmitted. The process is repeated starting with slot 1 of the next frame (not shown) resulting in 20 milliseconds worth of information being transmitted every 20 milliseconds. It should be noted that at the beginning of the process, only 10 milliseconds worth of information was transmitted. Also, it should be noted that at time t3 storage device 402 is empty and thus can be used again as it was used during the occurrence of slot 1 of the frame shown. The length of transmission for slot 1 or slot 4 is 6.67 milliseconds. There is an inherent delay associated with the interleaving process. The delay associated with the transmission of the contents of storage device 402 is 46.67 milliseconds (20 msec+20 msec.+6.67 msec.). The delay is defined as the maximum time that can elapse from the time of initially storing information in a storage device to the time the storage device becomes empty. The same process is followed by user 2 during slots 2 and 5 and user 3 during slots 3 and 6. It is in this manner that information of each user is spread out over two slots. For the half rate mode 2-interslot interleaving cannot be done within a single frame because only one slot is assigned per user during a frame. It should be noted that each buffer of storage device 402 as well as device 404 contain information that represents 20 milliseconds of speech which is transmitted within a time period of 6.67 msec. (length of a slot) because the speech is efficiently represented as mathematical parameters created by a vocoder.
The increasing use and popularity of wireless telecommunication systems mandate that such systems increase their user capacity. Due to improvements in communication technology equipment, more efficient vocoders, channel coders and modulators are being developed allowing system providers to transmit even more information in each slot. For example, modulators in the very near future should be able to compress three bits into one symbol. However, with the increased capacity comes a higher susceptibility to channel distortions. A user can lose a great amount of information due to channel distortions that affect an entire slot regardless of the channel coding technique used. A combination of different processing technique is needed to more effectively combat channel distortions. The use of combined processing techniques (e.g., channel coding combined with interleaving) many times depends on the format being followed by a system. For example, in IS-136 systems operating in the Half Rate mode, interslot interleaving is not possible because each user is assigned only one slot per frame. Therefore there exists a need to devise an information format that allows for the use of different processing techniques that can effectively attenuate the adverse effects of communication channel distortions.
The present invention provides a method for conveying information in a TDMA telecommunication system in which each user transmits and receives information during assigned system time slots within a time frame in accordance with a system format. The present invention describes a new format for conveying user and signaling information that improves the information and user capacity of the wireless communication system. The new format further allows more effective processing (e.g. inter-slot interleaving) to be used for protecting the conveyed information from channel distortions. In addition the new format is compatible with most features that can be implemented in current systems (e.g., all features in IS-136 compliant systems).
First, each time slot within each time frame is partitioned into a plurality of subslots and a subslot delineater field is created for each subslot where the plurality of subslots is time aligned with the system time slots. Each user is assigned to at least one subslot. Because the plurality of the subslots taken as a whole is aligned with the system time slots, the system has the flexibility of conveying information in accordance with the new format or current system formats.